Invest on the draft so that way you get your future trainees, which will be your future stars or future revenue.
Start off with competent staff under 15k, later on you can update to advanced
Leave your roster for only 11 players, play them in their roles and see which ones work better with your tactics and on your league.
once you start earning money start investing small sums into the arena, make sure you have high attendance, if not drop the prices